Download 5.1 Mendelian Genetics - Mrs. Mortier's Science Page

5.1
Mendelian Genetics
Understanding Inheritance
Genetics
Genetics is the study of inheritance. It studies the
mechanisms by which traits are passed between
parents and offspring.
The principles of genetics
were determined by Gregor
Mendel (1860s). Most of
his research came from
studying inheritance in
garden peas.
Characteristics (traits)of peas
studied by Mendel
Alleles
All traits are coded for in the DNA of an individual. The area on
the DNA that codes for a particular trait is called a gene.
From the diagram on the last slide, you may notice that for
every trait listed, there are 2 different versions.
These different versions of a gene are called alleles.
e.g. for flower colour, there are 2 alleles – purple and
white
Alleles
Each individual receives 2 alleles for every trait – one from the
mother and one from the father. The combination of alleles
that is inherited determines what characteristics an individual
shows.
Homologous chromosomes
Dominant traits
Dominant trait- is always expressed, if present. It masks a
recessive trait, if the recessive trait was also inherited in an
individual
Dominant allele (P - purple flowered pea plant)
a. homozygous dominant (PP – receives an
allele for the dominant trait from each parent)
b. heterozygous dominant (Pp – receives one allele for
the dominant trait and one for the recessive trait
(p-white flowers). The recessive trait is hidden by
the dominant trait, so this individual plant has
purple flowers)
Recessive traits
In order for an individual to express the recessive form of a trait,
they must inherit 2 alleles for the recessive trait from it’s
parents.
Recessive allele (p – white flowered pea plant)
To show white flowers, the plant must have be homozygous
recessive (pp).
Genotype
A genotype is the allele combination that an individual inherits
from its parents.
It is usually written as a letter code, where a capital letter
represents the dominant form of the allele, and a lower case
represents the recessive form of the allele.
PP – homozygous dominant
Pp – heterozygous
pp – homozygous recessive
Phenotype
Based on the genotype and the type of inheritance, the
individual will exhibit certain characteristics. This is called
their phenotype.
PP – purple flowered pea plant
Pp – purple flowered pea plant
pp – white flowered pea plant
**Note: a homozygous dominant and a heterozygous
individual both exhibit the same phenotype, even though their
genotypes are different.
Monohybrid Crosses
A monohybrid cross allows an experimenter to make a
prediction about the possible outcomes that can arise from
crossing 2 individuals of known genotype.
To do this, a Punnett square can be used to show how alleles in
parents segregate (in meiosis) to create gametes, and then
how these gametes potentially combine in fertilization.
Monohybrid Cross
• The experiment tracks the inheritance of a single trait.
• Mendel’s “Law of segregation”
a. pairs of genes separate during gamete
formation (meiosis).
b. the fusion of gametes at fertilization pairs genes
once again.
Law of Segregation
P
Pollen
P
P
Pp
p
p
diploid (2n)
p
meiosis I
meiosis II
haploid (n)
Monohybrid Cross
Example:
Cross between two homozygotes for purple flowers
(PP) and white flowers (pp)
PP = purple flowers
Pp = purple flowers
pp = white flowers
p
P
PP x pp
P
female gametes
p
male
gametes
Mendel’s P (parental) generation
Mendel took 2 true-breeding plants showing opposing
characteristics (purple flowered pea plant and white flowered
pea plant), and crossed them.
This is known as the P generation.
Punnett Square for the P generation
PP x pp
p
P
P
Pp
Pp
p
Pp
Pp
Analysis of the cross
PP x pp
p
P
Pp
p
Pp
F1 generation
P
Pp
Pp
Genotypic ratio: 1 Pp : 0
Phenotypic ratio: 1 purple: 0 white
Mendel then crosses 2 individuals
from the F1 generation
Mendel took 2 purple plants that he produced from crossing the
P generation (2 F1 generation plants), and crossed them.
Pp x Pp
F2 generation
GR: 1 PP: 2 Pp: 1 pp
PR: 3 purple: 1 white
The phenotypic ratio of this cross (a cross between 2
heterozygous parents) is known as the Mendelian ratio.